Variations in oceanic plate bending along the Mariana trench

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 401 (2014): 206-214, doi:10.1016/j.epsl.2014.05.032.We quantify along-trench variations in plate flexural bending along the Mariana trench in the western Pacific Ocean. A 3-D interpreted flexural deformation surface of the subducting Pacific Plate was obtained by removing from the observed bathymetry the effects of sediment loading, isostatically-compensated topography based on gravity modeling, age-related lithospheric thermal subsidence, and residual short-wavelength features. We analyzed flexural bending of 75 across-trench profile sections and calculated five best-fitting tectonic and plate parameters that control the flexural bending. Results of analysis revealed significant along-trench variations: the trench relief varies from 0.9 to 5.7 km, trench-axis vertical loading (-V0) from -0.73 × 1012 to 3.17 × 1012 N/m, and axial bending moment (-M0) from 0.1 × 1017 to 2.7× 1017 N. The effective elastic plate thickness seaward of the outer-rise region (Te M) ranges from 45 to 52 km, while that trench-ward of the outer-rise (Te m) ranges from 19 to 40 km. This corresponds to a reduction in Te of 21-61%. The transition from Te M to Te m occurs at a breaking distance of 60-125 km from the trench axis, which is near the outer-rise and corresponds to the onset of observed pervasive normal faults. The Challenger Deep area is associated with the greatest trench relief and axial vertical loading, while areas with seamounts at the trench axis are often associated with more subtle trench relief, smaller axial vertical loading, and greater topographic bulge at the outer-rise.This work was supported by US NSF Grant OCE-1141985 and Deerbrook Foundation (J.L.), NSF-China Grant 41376063 and Joint NSF China/Guangdong Natural Science Fund Committee U0933006 (W.Z.), and the Chinese Scholarship Council (F.Z.)

Intra- and intertrench variations in flexural bending of the Manila, Mariana and global trenches : implications on plate weakening in controlling trench dynamics

Author Posting. © Author(s), 2017. This article is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 212 (2018): 1429–1449, doi:10.1093/gji/ggx488.We conducted detailed analyses of a global array of trenches, revealing systematic intra- and intertrench variations in plate bending characteristics. The intratrench variations of the Manila and Mariana Trenches were analysed in detail as end-member cases of the relatively young (16–36 Ma) and old (140–160 Ma) subducting plates, respectively. Meanwhile, the intertrench variability was investigated for a global array of additional trenches including the Philippine, Kuril, Japan, Izu-Bonin, Aleutian, Tonga-Kermadec, Middle America, Peru, Chile, Sumatra and Java Trenches. Results of the analysis show that the trench relief (W0) and width (X0) of all systems are controlled primarily by the faulting-reduced elastic thickness near the trench axis (Tme) and affected only slightly by the initial unfaulted thickness (TMe) of the incoming plate. The reduction in Te has caused significant deepening and narrowing of trench valleys. For the cases of relatively young or old plates, the plate age could be a dominant factor in controlling the trench bending shape, regardless the variations in axial loadings. Our calculations also show that the axial loading and stresses of old subducting plates can vary significantly along the trench axis. In contrast, the young subducting plates show much smaller values and variations in axial loading and stresses.This work was supported by Chinese Academy of Sciences Grants (Y4SL021001, QYZDY-SSW-DQC005, YZ201325 and YZ201534), National Natural Science Foundation of China Grants (91628301, U1606401, 41376063 and 41706056) and HKSAR Research Grant Council Grants (24601515, 14313816)

Incision of submarine channels over pockmark trains in the South China Sea

The genesis of submarine channels is often controlled by gravity flows, but channels can also be formed by oceanographic processes. Using multibeam bathymetry and two-dimensional seismic data from the western South China Sea, this study reveals how pockmarks can ultimately form channels under the effect of bottom currents and gravity-driven sedimentary processes. We demonstrate that alongslope and across-slope channels were initiated by pockmark trains on the seafloor. Discrete pockmarks were elongated due to the erosion of gravity-driven sedimentary processes and bottom currents, and later coalesced to form immature channels with irregular thalwegs. These gradually evolved into mature channels with continuous overbanks and smooth thalwegs. Submarine channel evolution was significantly influenced by seafloor topography since the Late Miocene. The evolutionary model documented here is a key to understanding how channels are formed in deep-water environments

Quantitative and geomorphologic parameterization of megaclasts within mass-transport complexes, offshore Taranaki Basin, New Zealand

Mass-transport complexes (MTCs) in sedimentary basins reflect the gravitational transport of sediments from the shelf edge to the abyssal plain. Megaclasts, large sedimentary blocks of hundreds of meters long within MTCs, can record kinematic and sedimentary information deemed essential to understanding source-to-sink systems. Yet, deformation structures in such megaclasts remain poorly understood. This study uses high-quality, three-dimensional (3-D) seismic reflection data from the deep-water Taranaki Basin offshore New Zealand to analyze the morphological character of 123 megaclasts and propose a new classification scheme based on their morphometric properties. The megaclasts are up to 400 m tall, 1900 m long, and 1200 m wide. In the study area, they are high- to moderate-amplitude features owing to their different lithologies and continuous-to-contorted seismic facies. The megaclasts can be classified as undeformed, rotated, deformed, and highly deformed based on their internal deformational styles. Two different kinds of morphological depressions observed on their basal shear zones further indicate whether the megaclasts are transported or formed in situ. Our study demonstrates that quantitative parameterization of the megaclasts provides important information about their deformational processes and a more complete understanding of megaclast emplacement along continental margins

Influence of temperature, pressure, and fluid salinity on the distribution of chlorine into serpentine minerals

Serpentinization produces serpentine minerals that have abundant water and fluid-mobile elements (e.g., Ba, Cs, and Cl). The dehydration of serpentine minerals produced chlorine-rich fluids that may be linked with the genesis of arc magmas. However, the factors that control the distribution of chlorine into serpentine minerals remain poorly constrained. We performed serpentinization experiments at 80-500 degrees C and pressures from vapor saturated pressures to 20 kbar on peridotite, orthopyroxene, and olivine with < 5% pyroxene. The results show that the concentrations of chlorine in serpentine minerals were up to 1.2 wt% at 200 degrees C, whereas they decreased slightly at 311-400 degrees C and 3.0 kbar and became significantly lower at 485 degrees C and 3.0 kbar, similar to 0.1 wt%. Fluid salinity greatly decreased chlorine concentrations of olivine-derived serpentine produced at 400 degrees C and 3.0 kbar, which was associated with a decrease in silica mobility during serpentinization. By contrast, influence of fluid salinity at 311 degrees C and 3.0 kbar is minor. Moreover, chlorine distribution into serpentine can be influenced by primary minerals of serpentine. Serpentine formed in olivine-only experiments at 311 degrees C and 3.0 kbar had 0.08 +/- 0.03 wt% Cl, which is significantly lower than chlorine concentrations of serpentine minerals (0.49 +/- 0.36 wt%) produced in orthopyroxene-only experiments. By contrast, for experiments at 311 degrees C and 3.0 kbar, olivine- and orthopyroxene-derived serpentine had comparable amounts of chlorine. In particular, olivine-derived serpentine had 0.16 +/- 0.09 wt% Cl that was slightly higher than chlorine concentrations of serpentine formed in olivine-only experiments, whereas orthopyroxene-derived serpentine had significantly lower chlorine concentrations than that formed in orthopyroxene-only experiments. The contrast may be associated with releases of aluminum and silica from pyroxene minerals, which possibly results in a decrease in chlorine concentrations of serpentine. The concentrations of chlorine in serpentine formed in experiments at 311 degrees C and 3.0 kbar were slightly lower than those in serpentine produced at 300 degrees C and 8.0 kbar, which may be associated with influence of pressure on the mobility of iron and silica. The experimental results of this study indicate that serpentine minerals are important carriers of chlorine in subduction zones. It also suggests that chlorine is significant for the redistribution of cations during serpentinization

Influence of pyroxene and spinel on the kinetics of peridotite serpentinization

Hydrothermal experiments were performed at 311 degrees C and 3.0 kbar on natural olivine and peridotite to investigate the kinetics of serpentinization. The results show that the rates of reaction strongly depend on grain sizes of solid reactants, with smaller grain sizes resulting in faster kinetics. After 27 days of reaction, the reaction extent was 99% for peridotite with grain sizes of <30 mu m, and the reaction extent was 28% for grain sizes of 100-177 mu m. Compared to peridotite, olivine is serpentinized at much slower rates, e.g., 5.3% of reaction extent was achieved for olivine with grain sizes of 100-177 mu m after 27 days, approximately five times lower than that reached during peridotite serpentinization. Such contrasting results are due to the presence of pyroxene and spinel, an interpretation which is supported by a marked increase in reaction extents for experiments with the addition of pyroxene and spinel. The reaction extent achieved in experiments with 3 wt% spinel greatly increased to 98% after 27 days, much higher than that achieved during olivine serpentinization. These results appear to be related to pyroxene and spinel releasing Al and Cr during serpentinization. As indicated by compositions of serpentine, orthopyroxene lost similar to 60% of Al at a reaction extent of 59%. Influence of Al and Cr is suggested by a dramatic increase in reaction extents with the addition of Al2O3 and Cr2O3 powders. Olivine in natural geological settings is commonly associated with pyroxene and spinel; consequently, serpentinization kinetics may be much faster than previously thought

The production of iron oxide during peridotite serpentinization: Influence of pyroxene

Serpentinization produces molecular hydrogen (H2) that can support communities of microorganisms in hydrothermal fields; H2 results from the oxidation of ferrous iron in olivine and pyroxene into ferric iron, and consequently iron oxide (magnetite or hematite) forms. However, the mechanisms that control H2 and iron oxide formation are poorly constrained. In this study, we performed serpentinization experiments at 311 °C and 3.0 kbar on olivine (with <5% pyroxene), orthopyroxene, and peridotite. The results show that serpentine and iron oxide formed when olivine and orthopyroxene individually reacted with a saline starting solution. Olivine-derived serpentine had a significantly lower FeO content (6.57 ± 1.30 wt.%) than primary olivine (9.86 wt.%), whereas orthopyroxene-derived serpentine had a comparable FeO content (6.26 ± 0.58 wt.%) to that of primary orthopyroxene (6.24 wt.%). In experiments on peridotite, olivine was replaced by serpentine and iron oxide. However, pyroxene transformed solely to serpentine. After 20 days, olivine-derived serpentine had a FeO content of 8.18 ± 1.56 wt.%, which was significantly higher than that of serpentine produced in olivine-only experiments. By contrast, serpentine after orthopyroxene had a slightly higher FeO content (6.53 ± 1.01 wt.%) than primary orthopyroxene. Clinopyroxene-derived serpentine contained a significantly higher FeO content than its parent mineral. After 120 days, the FeO content of olivine-derived serpentine decreased significantly (5.71 ± 0.35 wt.%), whereas the FeO content of orthopyroxene-derived serpentine increased (6.85 ± 0.63 wt.%) over the same period. This suggests that iron oxide preferentially formed after olivine serpentinization. Pyroxene in peridotite gained some Fe from olivine during the serpentinization process, which may have led to a decrease in iron oxide production. The correlation between FeO content and SiO2 or Al2O3 content in olivine- and orthopyroxene-derived serpentine indicates that aluminum and silica greatly control the production of iron oxide. Based on our results and data from natural serpentinites reported by other workers, we propose that aluminum may be more influential at the early stages of peridotite serpentinization when the production of iron oxide is very low, whereas silica may have a greater control on iron oxide production during the late stages instead

Initiation and evolution of an isolated submarine canyon system on a low-gradient continental slope

Submarine canyons are important conduits transferring large volumes of sediment, nutrients, and pollutants from the continental shelf to deep-water basins. However, the mechanisms initiating submarine canyons and the factors influencing their evolution are still poorly understood. Here, we use multibeam bathymetry and two-dimensional seismic reflection data to investigate the origin and development of a submarine canyon system on the northern South China Sea margin. Our results show a submarine canyon system lying at a water depth of 400–1200 m on a relatively low-gradient (<0.5°), open continental slope. At the bottom of this canyon system, buried canyons undercut a mass-transport complex (MTC 1), whose top surface is early Pliocene in age. No other modern or buried canyons, channels and gullies are observed outside the area spanned by MTC 1. Such an observation demonstrates that pre-existing slide scars can capture gravity flows by providing accommodation space for sediment transported onto the lower continental slope, thus facilitating the development of pre-existing channels above MTCs. Lateral accretion packages identified on the southwest walls of several submarine canyons suggest they migrated northeastward due to the influence of contour currents. In addition, the presence of several basal erosional surfaces and smaller-scale MTCs in the canyons confirms they have undergone multiple cut-and-fill cycles, which were likely controlled by relative sea-level changes. The relative high sea level recorded at present ultimately led to the preservation of the studied canyon system on the continental slope. The results not only demonstrate the crucial role of submarine landslides in the initiation of submarine canyons, but also highlight how relative changes in sea level influence the evolution of submarine canyons on low-gradient continental slopes